Inositol hexaphosphate , calcium

Phosphate is better used when supplied by food than when supplied by phosphate salts, such as potassium phosphate. The food phosphate occurs largely as phosphate esters and, with gradual hydrolysis, enters the body relatively slowly and is used efficiently in the metabolic processes. Phosphate supplied as inorganic phosphate is rapidly absorbed, with a tendency to be excreted in the urine rather than to be used by tissues (Schuette and Linkswilcr, 1962). The phosphate in bran occurs largely in the form of phytic acid (inositol hexaphosphate). The phosphate groups of fhis compound may be only partially hydrolyzed in the gut- Phytic acid tends to be excreted via the fecal route as a complex with iron or calcium. 

FIGURE 10,42 Structures of components of the calcium signaling pathway Inositol hexaphosphate (phytic acid) appears not to be synthesized by mammalian cells. The phytic acid in the diet can, to some extent, be hydrolyzed to give inositol. Inositol is required in the diet of rodents but not of humans. 

Certain ions, e.g., oxalate and phosphate, can bind Ca within the intestinal canal and high intake of phosphate can restrict calcium absorption. A similar effect is encountered by phytic acid (inositol hexaphosphate) that is present in high amounts in certain cereals. Disturbances in the fat absorption (dietary) decreases the calcium absorption because Ca is bound to nonabsorbable fatty acids. 

Certain foods such as unrefined cereals and sugar cane juice contain organic phosphates, e.g. phytate (inositol hexaphosphate), which reduce enamel solubility, apparently by reacting with calcium phosphate salts on the enamel surface. The view that this type of compound has a caries-inhibiting effect is supported by observations that dietary supplements of calcium glycerophosphate reduce caries in rats and monkeys. At the same time, the calcium-binding properties of phytate may adversely affect the absorption of calcium from the intestine (page 143). 

Diets based on unleavened wheat bread contain a relatively large amount of phytic acid (inositol hexaphosphate), which can bind calcium, iron and zinc to form insoluble complexes that are not absorbed. Phytases in yeast catalyse dephosphorylation of phytate to products that do not chelate the minerals. 

Calcium is not always available in sufficient amounts from the diet because it is usually absorbed incompletely. Absorbability of dietary Ca++ depends greatly on the other food components. Oxalate and phytin (inositol hexaphosphate), in particular, impede the absorption in the small intestines by forming slightly soluble salts. Citrate, on the other hand, enhances the availability of Ca++, probably by forming soluble complex salts. Lastly, vitamin D greatly improves the absorbability of Ca++ the value of the vitamin in treating rickets is based, at least in part, on this effect. 

Plant foods contain relatively large amounts of inositol phosphates, including the hexaphosphate, phytic acid. Phytate chelates minerals, such as calcium, zinc, and magnesium, forming insoluble complexes that are not absorbed. However, both intestinal phosphatases and endogenous phosphatases (phytase) in many foods dephosphorylate a significant proportion of dietary phytate. The inositol released can be absorbed and utilized for phosphatidylinositol synthesis. 

Organic phosphorus compounds, primarily inositolhexaphosphates (probably more than 50% of all organic phosphates), occiu in soils. The parent cyclic polyol, inositol, exists in numerous stereoisomeric configurations, of which myo-, scyllo-, neo-, and cZZ-inositol have been isolated from soils as phosphate esters. The hexaphosphate of myoinositol (myo-IHP), phytic acid, occurs in plant tissues. It often occurs as phytin, the calcium magnesium salt. Esters of myo-IHP are readily adsorbed in acidic soil solution by clay minerals and finely divided hydrated oxides of iron and aluminum. Organic sulfur compounds present in soils probably occur primarily as amino acids—e.g., cysteine, cystine, and methionine. 

Phytic acid, myo-inositol hexakis (dIhydrogen phosphate), as-l,2,3,S-tnns-4,6-cyclohexanehexol-hexaphosphate a major phosphate storage compound in plants, which is especially abundant in oil seeds, legumes and cereal grains. It is the hexaphosphate of A/yo-inositol (see), in which each OH-group of myoinositol is esterified with phosphoric acid. Calcium and magnesium salts of P.a. are known as phytin. The commercial preparation of myo-inositol involves extraction of P. a. from com (maize) steep liquor, hydrolysis of the P.a. to myo-inositol and inorganic phosphate, and crystallization of the myo-inositol from water. 

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